The invention relates to gate valves and in particular to a valve for use on oil wells which will cut a wire line and seal in the well under emergency conditions.
During workover or testing of oil and gas wells, wire line operations are common. A cable passes through the wellhead for supporting and manipulating various tools and instruments.
If an emergency arises requiring the immediate shutin of the well, there is insufficient time to withdraw the cable. It is known to shear the cable so that the well may be closed in.
U.S. Pat. No. 4,215,749 issued Aug. 5, 1980 shows such a valve arranged to always shear the cable with a special shear surface near the upper edge of the gate. Since pressure exits below the valve, the upper side of the gate is the pressure sealing surface. Such shearing action leads to potential damage to the sealing surface during the shearing action. This valve gate has a tapered undercut. Should the cable be unloaded when sheared, the valve depends on gravity plus the tapered undercut to avoid jamming on the sheared end of cable. Since the tapered bottom edge is not well adapted to shearing, it presents a potential for jamming.
It is also known to use a single gate which simultaneously shears the cable at both the top and bottom surfaces of the gate. Again the upper surface is always used for shearing, leading to possible damage of the sealing surface. Substantial force is required because of the double shear.